1. Field of the Invention
This invention relates to automatic focusing devices for use in still cameras, video cameras, etc. and, more particularly, to automatic focusing devices in which a plurality of points in the picture frame are measured in distance, and focus adjustment information is formed on the basis of the results of distance measurement of these points.
2. Description of the Related Art
Devices of this kind for use in cameras, or devices of the so-called wide visual field distance measuring type (in which distance measurement is performed on a plurality of different points in the picture frame) have been proposed to eliminate the drawbacks of the conventional type automatic focusing device in which only one point of the picture frame is measured in distance. That is, that for a scene of two persons standing side by side, or a scene whose subject mainly a person) is out of the distance measuring area within the picture frame, these main objects fail to be sharply focused, as the focusing is effected at infinity.
What has so far been known as wide visual field distance measuring type automatic focusing device can be exemplified as follows:
(i) The type in which focusing is carried out based on one of the plurality of distance measuring results which is closest (for example, GENERAL & MECHANICAL 4582424, and Japanese Laid-Open Patent Applications Nos. Sho 61-88211 and;) PA1 (ii) The type in which if a plurality of distance measuring results lie within the field depth determined by the focal length and aperture size of the photographic lens, focusing is adjusted to the average value so that all the objects corresponding to the plurality of points of distance measurement are sharply focused (Japanese Laid-Open Patent Applications Nos. Sho 56-101128, 61-88211, 61-55619 and 61-53614.); PA1 (iii) The type in which depending on the brightness information, i.e., whether it is indoor or outdoor and, if outdoor, the focus is adjusted to the one of the plurality of distance measuring results which is farthest, or if indoor, it is adjusted to the closest one (Japanese Laid-Open Patent Applications Nos. Sho 61-55619 and 61-53614.)
But, the use of the above-described type (i) device leads to problem that for the scenes of a side by side arrangement of two persons and the corner location of a main object, although, in many cases, automatic focusing is effected on the person (the main object), if an obstacle lies in front of it, or there is a foreground, the photographic lens will be focused on these surroundings. So, it cannot be focused on the person (the main object).
In connection with the type (ii) device, it is natural that photographic situations where the measured values of distances to the all the points fall in a range for the field depth are very few in number except when a short focal length lens is used, or the ambient brightness is very high. At the time of using the long focal length lens, or of low brightness conditions, a drawback is invited in that the influence of the hand shake appears in the image, or the lens cannot be focused to any of the plurality of previously selected distances.
Further, in the above-described Japanese Laid-Open Patent Application No. Sho 56-101128 that discloses the type (ii) device, an attempt has been made to eliminate the above-described drawback even when the long focal length lens is in use, or the brightness of the object to be photographed is low. To this end, from the AE information for a given time, how deep the field depth is determined. This value of the field depth is then compared with the position of the object (the measured value of the object distance). In the case when it is found that all measured distance information does not fall within this field depth, a treatment to give an alarm to the photographer or to stop the shooting is carried out. But, the introduction of such an treatment gives rise to an alternative problem that a shutter opportunity is missed or the like.
Furthermore, the countermeasure in the type (iii) device is so short that in outdoor photography the sharp focus is apt to be effected on the background, while in indoor photography a front obstacle, etc. is often chosen in focusing, thus inviting a problem that the percentage of photographs of the person (main object) sharply focused is largely reduced.
This, in the automatic focusing device of the wide visual field distance measuring type according to the previous proposals, most of the photographic situations where two persons at the center of the target area stand side by side, or a person lies in the margin of the target area allow sharp focusing of the person as the main object). On the other hand, where the conventional distance measuring type (in which only one point of the picture frame is measured) is able to focus the lens on the main object, the focusing fails (instead, the lens is focused on the foreground or background.)